This invention relates generally to the art and science of wastewater treatment, and more particularly, to a process and system for optimising the energy balance associated with the energy consumed during downstream processing of sludge from the wastewater, as compared to the energy yield, and reduction of solids mass associated with such processing.
Industrial and municipal entities treat wastewater to prevent the contamination and pollution of local receiving waters and potable water supplies. Such treatment facilities are designed to remove inorganic and organic pollutants from the wastewater using various biological aerobic and anaerobic processes.
In general, industrial and municipal entities incur substantial costs in the operation of these wastewater treatment facilities. In addition to utility costs to operate the necessary machinery and mechanical systems, a facility also typically incurs substantial costs for the disposal of waste sludge generated by the various treatment processes. Sludge produced during wastewater treatment includes primary sludge from the pre-purification stage and biologically activated sludge from aerobic digestion. Stabilised sludge may be produced through the subsequent application of anaerobic digestion of biologically activated sludge with or without the addition of primary sludge. In some wastewater treatment facilities, these sludges are disposed by incineration, landfill, or spread as fertilizer over agricultural fields. All of these disposal methods result in expensive costs to the facility. Based on these substantial operational and disposal costs, it would be desirable to optimise the energy consumption for processing the wastewater and sludge to attain an improved quality of wastewater discharge and/or reduction in sludge disposal costs.
Anaerobic digestion is a microbiological process in which organic materials are broken down by the action of microorganisms in the absence of oxygen. The anaerobic microorganisms reduce the quantity of organic matter present in the biologically activated sludge thereby generating bio-gas having a relatively high methane gas content. The stabilised sludge is typically removed from a digestion tank for dewatering and disposal. The methane gas can be burned off or recovered to supply energy to heat the digesters as well as supply energy for use elsewhere in the treatment facility.
In dewatering processes, water is mechanically squeezed or separated from the sludge stream. Most advancements in this field of technology have sought to optimise the energy consumed in processing the sludge with the reduction in the volume of sludge disposed. Additionally the disruption technologies have sought to optimise the mass reduction of sludge for disposal.
An overview of conventional disruption methods can be found in a publication by N. Dichtl, J. Mxc3xcller, E. Engelmann, F. Gxc3xcnthert, M. Osswald entitled, Desintegration von Klxc3xa4rschlammxe2x80x94ein aktueller xc3x9cberblick in: Korrespondenz Abwasser, (44) No. 10, pp. 1726-1738 (1997). This publication describes three mechanical disruption techniques: (1) stirred ball mills; (2) high-pressure homogenisers; and (3) ultrasonic homogenisers. With the aid of these disruption methods, the micro-organisms and particulate solids in sludge are essentially comminuted or chopped-up. For example, the cellular walls of microorganisms and particulates present in sludge may be destroyed when the external pressure exceeds the cell internal pressure with the use of a homogeniser. The cell contents, which are separated from the exterior by the cell wall, are thereby released and become available for subsequent digestion.
An advantage of these disruption processes when applied to sludge is that the anaerobic micro-organisms are also disrupted together with the aerobic micro-organisms, in contrast to other methods in which such micro-organisms at least partly survive the disruption process. They remain in the disposed sludge as organic residue. A second advantage of disruption is that organic substances contained within the cellular contents of the sludge are released to the micro-organisms during the disruption process. In this way, they serve as internal sources of carbon to support de-nitrification in the digestion process.
Another publication concerning disruption of primary sludge using ultrasonic homogenisers is described in G. Lehne, J. Mxc3xcller: xe2x80x9cThe Influence Of The Energy Consumption On The Sewage Sludge Disruption,xe2x80x9d Technical University Hamburgxe2x80x94Harburg Reports On Sanitary Engineering, No. 25, pp. 205-215 (1999). The Lehne et al. publication describes that cell disruption is greater when the amount of cavitation bubbles in the vicinity of an ultrasonic probe is higher. The amount of cavitation bubbles is proportional to the intensity of the ultrasonic probe. Further study of the optimisation of the ultrasonic probe intensities was necessary in order to optimise the energy balance. A comparison of the ultrasonic homogeniser with high-pressure homogeniser and ball stirring mill provided comparable results in this process. However, mechanical problems, due to coarse material, occurred in the high-pressure homogeniser and ball stirring mill.
Disruption of the organic content in stabilised sewage sludge is also described in H. Grxc3xcning: xe2x80x9cEinfluss des Aufschlusses von Faulschlxc3xa4mmen auf das Restgaspotential.xe2x80x9d This article describes that, in processing anaerobic stabilised sewage sludge, gas production is considerably increased by prior disruption using ultrasound. An article by J. Mxc3xcller, N. Dichtl, J. Schwedes, xe2x80x9cKlxc3xa4rschlammdesintegrationxe2x80x94Forschung und Anwendungxe2x80x9d, Publication of the Institute for Settlement Water Economy of the Technical University Braunschweig, No. 61, Conference on the 10th and 11th of March 1998 in Braunschweig, pp. 180-191 (March 1998) discloses the use of a high-pressure homogeniser to disrupt stabilised sludge at pressures in the range of 500 to 1000 bar. Accordingly, they subscribe to conventional wisdom, which dictates that increased homogeniser pressures should be employed in order to increase the degree of disruption of the microbial sludge cells. Under this assumption, the amount of cell disruption increases in proportion to the degree of energy input. Accordingly, attempts so far have generally been directed to the application of disruption and/or anaerobic digestion of unconcentrated biologically activated sludge to reduce volume which has to be disposed.
A general description of the effects of sludge concentration and disruption of stabilised sludge can be found in T. Onyeche, O. Schlxc3xa4fer, H. Klotzbxc3xccher, M. Sievers, A. Vogelpohl: xe2x80x9cVerbesserung der Energiebilanz durch Feststoffseparation bei einem kombinierten Verfahren aus Klxc3xa4rschlammdesintegration und Vergxc3xa4rung,xe2x80x9d DechemaJahrestagungen 1998, Volume II, pp. 117-118 (1998). This article teaches that the sludge solids content can be concentrated using a decanter and thereafter homogenized. However, the high-pressure homogenisers used in this reference are operated at pressures of at least 500 bar. In any event, this article fails to adequately solve problem of optimising the energy balance of the system.
U.S. Pat. No. 6,013,183, which issued Jan. 11, 2000, discloses the application of high pressure homogenisation to biologically activated sludge. The sludge is homogenised at a pressure drop in excess of 5000 PSI (350 bar) across the homogenisation nozzle as a means of improving the reduction of volatile total solids when the liquefied biological activated sludge is recycled back to the aerobic digester. The patent also discloses the application of high pressure homogenisation of biologically activated sludge prior to anaerobic digestion, but it fails to address what, if any, treatment should be applied to primary sludge, or to further processing of stabilised sludge. Moreover, the issue of achieving a positive energy balance, such as through prior concentration of the sludge, is not addressed.
U.S. Pat. No. 4,629,785, which issued Dec. 16, 1986, disclosed the application of high pressure homogenisation to both biologically activated sludge and stabilised sludge at pressures of up to 12,000 PSI (825 bar) prior to recovery of proteins in the sludge. This patent similarly excludes treatment of primary sludge and does not address energy recovery through production of methane gas during anaerobic digestion of the liquefied sludge.
Notwithstanding the above-described methods for treating sludge, a need for optimising the energy balance of the disruption process to minimize energy costs exists. The possible benefit of concentrating sludge prior to homogenisation has not previously been disclosed. In optimising the energy balance, it would be desirable to determine when the energy required to disrupt and otherwise pretreat primary and/or secondary sludge is about the same as, or considerably lower than, the energy obtained through additional methane gas yield. In this regard, it would also be desirable to optimise the disruption process in such a manner that the methane gas produced during the sludge digestion processes can be used as a source of energy to self-sustain the disruption process as well as other treatment processes. Accordingly, there is a need for a wastewater treatment system that positively balances the energy required to disrupt a sludge stream with the energy yield due to an increased production of methane gas (which can be converted to electrical energy).
Accordingly, it is a general object of the invention to overcome the deficiencies in the wastewater treatment art.
It is another object of this invention to optimise the energy balance associated with the energy consumed for processing of sludge produced during wastewater treatment and the energy yield obtained from the increased production of methane gas during anaerobic digestion of sludge.
It is a further object of this invention to provide a system and method that disrupts cellular walls of micro-organisms present in stabilised sludge in order to release nutrients for enhancing the sludge digestion process, and thus reducing the mass of stabilised sludge which must be disposed.
These and other additional objects and advantages are achieved in a unique combination of methods and systems for treating sludge generated in a waste water treatment facility according to the present invention. The method comprises increasing the solids concentration in primary, biologically activated, and stabilised sludge or any mixture thereof which undergoes anaerobic digestion. In a preferred embodiment, a homogeniser, operating within an economically viable low-pressure range, then disrupts the cellular walls of the various micro-organisms in the concentrated sludge, thereby releasing nutrients from within the cells. Disruption can occur either continuously or discontinuously. The disrupted sludge is subsequently supplied to a digestion tank, providing additional nutrients to enhance the production of methane gas. In this way, the invention optimises the energy demand in concentrating and homogenising the sludge as compared with the energy yield from the increased production of methane gas generated during the digestion process.
In accordance with one aspect of the invention, a positive energy balance is achieved with the use of a concentrated sludge having a high solids concentration that is processed under a reduced homogeniser pressure. The sludge is preferably concentrated by a factor of about 1.5 or greater prior to being processed with a homogeniser. Also, the homogeniser is preferably operated at a low-pressure range of less than 400 bar. In this range, the disruption step operates self-sufficiently, and even provides excess energy. This invention ascertained that the high-pressure homogeniser should advantageously be operated at a pressure of 50 to 400 bar, where the optimum is at the lower range of 100 or 200 bar. As explained below, even lower pressures may be achieved with the use of particular equipment, such as the APV Micro-Gap or Super Micro-Gap range of homogeniser valves.
In accordance with one alternative feature of the invention, the sludge undergoes a classification process prior to disruption. In this way, solid material particles are removed from the sediment sludge before they reach the homogeniser. The efficiency of homogenisation is improved in this manner. For example, classification of the sludge can take place with the use of a wet sieve device or sieve.